1. Field of the Invention
The present invention relates to a protective circuit for a secondary battery pack and a method of operating the protective circuit. More particularly, the present invention relates to a protective circuit and method that shifts the output voltages of one or more controllers such that each of the controllers in the circuit has approximately the same voltage output thus providing a simple protective circuit configuration while maintaining the safety of the banks in the circuit.
2. Description of the Related Art
In a conventional secondary battery, for example, in a lithium ion battery or a lithium polymer battery, each cell has a positive electrode plate, a negative electrode plate and a separator wound into a jelly roll. Each cell may output a voltage of about 4 volts (V). In the case of laptop computers or other electronic appliances that consume a large amount of electric power, a number of cells are coupled in series or in parallel to obtain an output voltage of about 16V to about 32V. When coupled in parallel, the cells may have a capacity of thousands of milliamp hours (mAh). A number of laptop computers contain secondary batteries configured with a group of four cells coupled in series, wherein each of the cells is coupled in parallel to another cell that is not one of the four cells in series, i.e., 4S1P mode. Each configuration of cells is a bank. A number of banks each having 4S1P mode may be coupled in series or in parallel. As is well known in the art, if a bank has an output voltage of about 16V, two banks coupled in series will have a total output voltage of about 32V.
Each bank has a protective circuit that has a controller coupled with the bank to protect the bank from overcharging or from over-discharging voltage. For example, conventional field effect transistors and/or fuses are installed on a large-current line and are cut off, thereby reducing the current flow to and from the bank, when the bank is overcharging or over-discharging, respectively.
When a number of banks are coupled in series, each bank outputs a different voltage. Since a controller senses the amount of charging or discharging voltage of a bank, and outputs a corresponding voltage, each controller disadvantageously outputs a different voltage. Accordingly, the number of field effect transistors or fuses must disadvantageously correspond to that of the controllers which control them. Further, field effect transistors or fuses coupled with controllers may need to be designed with different specifications due to different output voltages from the controllers.